The present invention relates generally to field-type electron emitters, and, more particularly, to a system and method for limiting the effects of arcing in field-type electron emitter arrays. By including a resistive substance in the gate layer of an emitter array, arc current through a given emitter can be limited and neighboring emitters can maintain electron emission. A more robust field emitter array is thus achieved.
Electron emissions in field-type electron emitter arrays are produced according to the Fowler-Nordheim theory relating the field emission current density of a clean metal surface to the electric field at the surface. Most field-type electron emitter arrays generally include an array of many field emitter devices. Emitter arrays can be micro- or nano-fabricated to contain tens of thousands of emitter devices on a single chip. Each emitter device, when properly driven, can emit a stream or current of electrons from the tip portion of the emitter device. Field emitter arrays have many applications, one of which is in field emitter displays, which can be implemented as a flat panel display. In addition, field emitter arrays may have applications as electron sources in microwave tubes, x-ray tubes, and other microelectronic devices.
The electron-emitting field emitter devices themselves may take a number of forms. FIG. 1 depicts an example of a common type of field emitter 10 known as a “Spindt”-type emitter. Emitter 10 includes a conductive substrate 12, which is often a heavily doped silicon-based substance. On the substrate 12 is grown a layer of silicon dioxide (SiO2) 14, to act as an insulator. A metal film 16, usually of molybdenum (Mb), is laid over the silicon dioxide 14, to form a conductor-insulator-conductor cross-section. Typically, the metal layer 16 is etched to form a hole 22 therethrough, and the silicon-dioxide 14 is dissolved to form a cavity 20 into which a emitter cone or tip 18 is placed. Emitter tip 18 is typically also formed of molybdenum.
In operation, a control voltage 24 is applied across metal layer 16 and substrate 12, creating a strong electric field near opening 22. Thus, metal layer 16 acts as a gating electrode for the emission of electrons from emitter tip 18. Typically, metal layer 16 is common to all emitters of an emitter array and supplies the same control or emission voltage to the entire array. In some Spindt emitters, the control voltage may be about 100V. Because of the conical shape of emitter tip 18, the interaction of the tip 18 and the electric field near opening 22 is focused at a smaller point and electron emission 26 is more easily achieved. However, many other shapes and types of emitter cones or tips may be used in Spindt emitters and other emitter device types. Other types of emitters may include refractory metal, carbide, diamond, or silicon tips or cones, silicon/carbon nanotubes, metallic nanowires, or carbon nanotubes.
At present, field emitter arrays are not known to be robust enough for use in several potential commercial applications, such as for use in x-ray tubes. Many existing emitter array designs are susceptible to operational failures and structural wear from electrical arcing. Arcing may be more likely to occur in the high pressures which exist in many x-ray tubes. Most commonly, an overvoltage applied to metal layer 16 of the emitter 10 of FIG. 1 may cause an arc to form between the metal layer 16 and the emitter tip 18, permitting current to flow in a short circuit from the metal layer 16 through the emitter tip 18 to the substrate 12. Another type of arcing is known as surface flashover arcing, in which an overvoltage applied to metal layer 16 can cause a breakdown of the silicon dioxide insulating layer 14 which allows current to punch through, creating a short circuit between the metal layer 16 and substrate 12. The arc can also pass over the surface of the silicon dioxide insulating layer, resulting in what is known as a “flash over”
When one emitter of an emitter array experiences arcing in either form, or “breaks down,” the metal layer will no longer be able to support a voltage or electrical bias sufficient for electron emission to continue at the other emitters of the array. In addition, high temperatures produced by the short circuit current can cause wear or damage to the emitter as well as neighboring emitters. Thus, an arc at one emitter can affect the operation of the entire emitter array.
It would therefore be desirable to have a system and method which protect an emitter array from the effects of arcing. It would be further desirable for such a system and method to protect both the operation and structure of the array by maintaining the emission or control voltage at non-arcing emitters and limiting the arc current of the arcing emitter.